Heterodyne receivers are widely used in communications and other equipment for converting applied input signals to fixed lower frequency signals. By means of such receivers, an input signal with frequency components that vary over a wide frequency range may be converted to a fixed frequency signal (usually referred to as an intermediate frequency signal) for subsequent detection and measurement. A swept heterodyne receiver of the prior art is shown in FIG. 1. Generally, the greater the number of oscillators used in the receiver to perform the conversion, the more selective the receiver becomes.
To correct for factors such as frequency drift, frequency jitter (residual FM noise) and phase noise which limit the frequency resolution of the receiver, a selected oscillator, typically the first oscillator of the receiver, is phase locked to a spectrally pure reference signal (i.e., a reference signal with a noise level lower than the signal level desired to be detected and measured) by means of a phase lock loop as shown in FIG. 2. Such a phase lock loop serves to correct for residual FM and phase noise introduced into the receiver system by the first oscillator (a voltage controlled oscillator), but does not correct for (i.e., does not substantially eliminate) residual FM and phase noise introduced into the system by the other oscillators of the system (e.g., the second through the n.sup.th oscillators).
Also, it is often difficult to determine the tuned frequency of the receiver (i.e., the exact frequency to which the receiver is tuned). This difficulty arises from the fact that in some very wide tuning receivers, such as spectrum analyzers, many of the oscillators are tuned to microwave frequencies (e.g., 2-4 GHz), which frequencies are difficult to count directly. (It is necessary to count the frequency of each and every one of the oscillators in order to find the tuned frequency of the receiver, because the tuned frequency of a receiver is dependent upon the frequencies to which the individual oscillators are tuned.)
What is needed, therefore, is a system that corrects for frequency drift, residual FM, and phase noise from all of the oscillators in the system, and that provides a means for determining the exact frequency to which the system is tuned without having to count the frequencies of the individual oscillators.